FIG. 1 shows the structure of a conventional satellite transmission apparatus. As shown, the conventional satellite transmission apparatus mainly includes a receiving antenna 1′, an RF amplifying system a′, two band-pass filters 2a′, 2b′, two mixers 3a′, 3b′, two local oscillators 4a′, 4b′, a high-pass filter 5′, a low-pass filter 6′, and a multiplex adder 7′.
The receiving antenna 1′ receives vertical and horizontal polarized signals from a satellite. The received vertical and horizontal signals are down-converted and amplified at the RF amplifying system a′, and then filtered at the band-pass filters 2a′ and 2b′, respectively. Frequency subtraction operations are then executed at the mixers 3a′ and 3b′ to subtract frequencies of the filtered vertical and horizontal signals from oscillating frequencies of the local oscillators 4a′ and 4b′, respectively. Signals output from the mixers 3a′ and 3b′ are filtered at the high-pass and the low-pass filters 5′ and 6′, respectively, to filter and isolate noises therefrom to generate two different and non-repeated mid-bands. The two mid-bands are added at the multiplex adder 7′. However, second and third harmonic differences obtained from the oscillating frequencies of the two local oscillators 4a′ and 4b′ separately fall in the ranges of these two mid-bands to result in intermodulation interference.
To enable better understanding of many drawbacks of the above-described conventional satellite transmission apparatus of FIG. 1, an example thereof is now described in more details.
Please refer to FIG. 1. The receiving antenna 1′ receives a satellite vertical polarized signal having a frequency within the range from 11.7 to 12.2 GHz, and a satellite horizontal polarized signal also having a frequency within the range from 11.7 to 12.2 GHz. These signals are filtered at the band-pass filters 2a′ and 2b′, respectively. The local oscillators 4a′ and 4b′ have oscillating frequencies of 10.75 GHz and 10.15 GHz, respectively. The frequency of the filtered vertical signal is subtracted from the oscillating frequency of the local oscillator 4a′ to obtain a maximum vertical frequency of 1450 MHz (i.e., 12.2 GHz−10.75 GHz=1.45 GHz=1450 MHz) and a minimum vertical frequency of 950 MHz (i.e., 11.7 GHz−10.75 GHz=0.95 GHz=950 MHz) for a first mid-band, as shown in FIG. 2A. Meanwhile, the frequency of the filtered horizontal signal is subtracted from the oscillating frequency of the local oscillator 4b′ to obtain a maximum horizontal frequency of 2050 MHz (i.e., 12.2 GHz−10.15 GHz=2.05 GHz=2050 MHz) and a minimum horizontal frequency of 1550 MHz (i.e., 11.7 GHz−10.15 GHz=1.55 GHz=1550 MHz) for a second mid-band, as shown in FIG. 2B. Signals within the frequency ranges of the above two mid-bands are then filtered and isolated at the high-pass filter 5′ and the low-pass filter 6′, respectively, to generate two mid-bands, which are combined at the multiplex adder 7′, as shown in FIG. 2C. The second harmonic difference between the local oscillators 4a′ and 4b′ is 2×10.75 GHz−2×10.15 GHz=1.2 GHz=1200 MHz; and the third harmonic difference between the local oscillators 4a′ and 4b′ is 3×10.75 GHz−3×10.15 GHz=1.8 GHz=1800 MHz. These two harmonic differences separately fall in the ranges of the two resultant mid-bands, as shown in FIG. 2D, to cause intermodulation interference.